MB-7459 should count expired items during warmup

For better account of expired items, numExpiredItems must
also count any item that has expired (and deleted) during
the data loading phase of warmup.

Unit test case that requires memcached test api will be
merged with a separate patch

Change-Id: Ia1b8cce6daf29c5bca83179c63bfeb1e3e7c2be0
Reviewed-on: http://review.couchbase.org/23934
Tested-by: Jin Lim <jin@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>